1. Field of the Invention
This invention relates to a method of eliminating water vapor interference in detecting gases wherein absorption of infrared light is used to determine measured absorption and interference absorption.
2. Description of Prior Art
Utilities which distribute natural gas require reliable, portable gas-leak detectors for use in maintenance of gas supply lines. Existing natural gas detectors are either costly, sensitive and non-selective or low cost, insensitive and non-selective. Non-selective gas detectors respond to any combustible gas. Selective gas detectors are specific to hydrocarbon gases. The two most presently used detectors are based on hydrogen flame ionization and on hot wire catalysis which cannot distinguish among different types of hydrocarbons. However, it is necessary to distinguish among different types of hydrocarbons in order to distinguish a pipeline gas from gasoline vapors or sewer or swamp gas and to reduce leak surveyor time wasted on false alarms. Ethane content, if measurable, provides a good means to discriminate between pipeline gas and interfering sewer or swamp gases because the latter contain practically no ethane, while pipeline gas does, in varying degrees. Gasoline vapors and propane (LP gas) can also generate a false alarm with conventional instruments. However, their infrared absorption is shifted relative to that of methane, as will be described later, it is the basis for this invention to eliminate interference caused by water vapor and to eliminate sending false alarms.
Many methods have been developed to eliminate undesirable interferences in gas sensors. However, existing infrared absorption band gas detectors are unable to eliminate interference caused by water vapor in the gas being detected. This water vapor interference can be caused by naturally occurring humidity in the air or the environment in which the detector is being used.
U.S. Pat. No. 4,507,558 discloses a selective detector for natural gas which discriminates between low concentrations of natural gas and other methane sources by measuring the characteristics of the methane/ethane ratio of natural gas as well as by using a combustible gas sensor. The operation of this detector is based on infrared light absorption of methane and ethane in combination with another non-specific combustible gas detector whereby the detector has the ability to detect non-specifically, the presence of a combustible gas, and to define the nature of the combustible gas. Thus, this natural gas detector utilizes two types of detection including non-dispersive infrared detectors and a non-specific combustible detector such as a hot-wire catalytic combustible detector. The detector determines concentration of the other gas by using absorption cells placed in front of the detectors. The detector includes a light emitting diode which issues light centered around 3.32 microns and a reference light source which emits light at a wavelength outside of this band. Although this arrangement permits distinguishing among different types of hydrocarbons, the requirement for a hot-wire catalytic combustible detector adds cost and complexity to the device and increases power consumption.
U.S. Pat. No. 3,893,770 teaches a multi-channel gas analyzer which decreases interferences among the absorption spectra for component gases of mixed gases. The '770 patent discloses an apparatus which uses a spectrophotometer to detect an absorption spectra for each of three types of gases and further uses a single measurement device to determine the concentrations of the three types of component gases. A dispersive spectrometer is used to obtain absorption spectra of specific wavelength ranges corresponding to the component gases which make the interferences between the component gases irreversible. The absorption spectra are converted to electrical signals and the concentrations of the component gases are measured by compensating for irreversible interferences by means of function generations and arithmetic units each of the numbers of which are equal to that of the irreversible interferences.
U.S. Pat. No. 4,054,384 discloses an infrared radiation source which passes infrared radiation through a cell to a conventional Michelson-type analyzing interferometer and the resultant radiation proceeds to a detector, which emits an electrical signal. The electrical signal is split into two signals, one signal passes through a time delay device in route to an input amplifier, and the other signal passes directly to the input amplifier. The differential amplifier operates in conjunction with a delay device to provide autocancellation of signal components due to contaminants in the sample gas stream.
U.S. Pat. No. 3,897,154 discloses a method of determining unknown constituents and measuring constituent concentrations in a fluid while a contaminant or extraneous constituent of the fluid interferes with monitoring the constituents sought. The '154 patent teaches that the unknown constituents, of the fluid, flow in a path so as to influence the time delay between the arrival of each sample constituent at two stations. The '154 patent teaches a substantial delay, compared with the effective residence time of the stream at the last station, the station which the sample arrives at first. The two samples from the respective measurement stations may be compared with each other and interrupted in various ways, including but not limited to subtraction of one signal from the other. Thus the two signals may provide contaminant-effect autocancellation and may be equal.
U.S. Pat. No. 4,273,450 discloses determining characteristics of unknown samples with a photoacoustic spectrometer. An electrical signal obtained from an unknown-sample unit is cross-correlated with an electrical signal from a reference-sample unit to produce a third signal which is used to identify the unknown sample.
It would be highly desirable to have a natural gas detector which can positively distinguish among different types of hydrocarbons by eliminating absorption caused by water vapor, and can provide information to the operator on the amount and type of combustible gases in the environment.